Circuit breakers are widely used to protect electrical lines and equipment. The circuit breaker monitors current through an electrical conductor and trips to interrupt the current if certain criteria are met. One such criterion is the maximum continuous current permitted in the protected circuit. The maximum continuous current the circuit breaker is designed to carry is known as the frame rating. However, the breaker can be used to protect circuits in which the maximum continuous current is less than the circuit breaker frame rating, in which case the circuit breaker is configured to trip if the current exceeds the maximum continuous current established for the particular circuit in which it is used. This is known as the circuit breaker current rating. Obviously, the circuit breaker current rating can be less than but cannot exceed the frame rating.
An electronic trip unit (“ETU”) is a device that is used in conjunction with an electro-mechanical circuit breaker to control the current versus time trip response (also voltage). The current versus time trip characteristics are, in part, a function of the maximum continuous current permitted by the circuit breaker. This maximum continuous current is also called the current rating of the circuit breaker. As long as the current remains below this maximum continuous current rating, the breaker will remain closed. Momentary low magnitude excursions above the rated current are tolerated; however, persistent overcurrent conditions result in tripping of the breaker. The time delay and generation of the trip signal is an inverse function of the magnitude of the current. For very large magnitude overcurrents, such as would be produced by a fault, the microcontroller is programmed to generate a trip signal instantaneously.
The modification of the current versus trip time response curve is a serious matter. For safety purposes, the circuit breaker and trip unit combination must be properly configured to provide the type of protection judged by the customer or plant engineer to be appropriate. Therefore, the modification to this protection must also be considered to be a very serious event and handled in a way that prohibits errors.
Typically the breaker current rating is defined in two parts. The current sensor installed in the breaker has a rating less than or equal to the frame rating of the breaker. This is referred to as the breaker Sensor Rating. The current rating is further modified by installation of a rating resistor, which is selected to generate a preset voltage when a current proportional to the maximum continuous current permitted in the protected circuit passes through the rating resistor. In order to provide for adjustment of current rating so that the circuit breaker can be used to protect circuits with different maximum continuous currents, it is known to incorporate the rating resistor in a replaceable rating plug, which may be selectively inserted in to the breaker.
Electronic trip circuit (ETU) interrupters are designed to interrupt overcurrent conditions over a wide range of ampere rating. The current through the protected electric power circuit is continuously sensed by means of current transformers and a voltage signal is supplied to the signal processor within the ETU circuit A common electronic circuit interrupter can operate over a wide range of ampere ratings by merely changing the rating plug. It is important to prevent the insertion of an electronic circuit into a circuit interrupter which results in a current rating greater than the sensor rating or frame rating. This can result in a condition where the electrical distribution circuit is not protected. Similarly, it is important not to insert an electronic circuit into a circuit interrupter that results in an unintentional low trip level that allows so-called “nuisance tripping” to occur. Finally, standards require that a circuit interrupter with a replaceable current rating (like a rating plug) either do not close or trip at or below the lowest published current trip level if the rating plug is not inserted.
Field replaceable rating plugs are known. These rating plugs are field installable and may be mechanically configured for use with thermal-magnetic trip units or may use a combination of analog circuit scaling and digital techniques to change the ETU response. It is typical for ETU housings to provide mechanical rejection of plugs that are not suited to certain ranges or frame sizes.
A typical method to prevent incompatible ETU/rating plug combinations includes a first manufacturing process of providing interlocking pins that can be mechanically modified by a secondary manufacturing process of breaking out pieces. The secondary manufacturing process breaks out small pieces of plastic on the housing of the rating plug and complementary pieces on the housing of the ETU.
Current sensors are typically installed as part of the circuit breaker during manufacture. A unique identifying number is assigned to the circuit breaker, which defines the frame rating and sensor rating. The electronic trip unit is configured at the time of manufacture to indicate the frame and sensor ratings of the circuit breaker or circuit breakers with which it is compatible. A unique identifying part number is assigned to the configured trip unit. A specifying engineer orders a specific combination of trip unit and circuit breaker to satisfy the requirements of the power system installation. Appropriate combinations are enforced through mechanical or electronic rejection.
A problem associated with mechanical rejection of plugs and trip units is the cost associated with the secondary operation and the limitation of the number of combinations that can be rejected. In some cases the mechanical rejection method is not reliable because some operators, using great force, can insert an incorrect rating plug or install an incorrect trip unit.
Manufacturers also use mechanical rejection in the interface between the trip unit and the circuit breaker mounting point for the trip unit. Trip units are configured, in part, to match the characteristics of the underlying circuit breaker's sensor rating, presence or absence of additional sensors, frame rating, and breaker type. Rejection methods similar to those described for rating plugs are employed to ensure that only a properly matched trip unit can be successfully installed to a circuit breaker. Similarly, mechanical rejection means may be overcome by the application of excessive force, resulting in an invalid and potentially unsafe configuration.
In use, electronic trip units may be exchanged from one circuit breaker to another during the course of maintenance of a power distribution system, or when upgrading the trip unit in a breaker that has been in service for several years, an activity known as “retrofitting”. Newly designed trip units are often required to maintain “backwards compatible” mechanical and electrical interfaces to existing trip systems, sometimes several different trip systems, which adds cost and complexity to new designs.
When retrofitting a new trip unit to an older circuit breaker, the mechanical rejection means employed by the circuit breaker must be carried through to the new trip unit. If the trip unit is intended for use in several different breaker products the number of rejection permutations can be unmanageably large. The specifying engineer may need to properly identify not only the correct breaker, trip unit, and rating plug combinations, but also an appropriate ‘retrofit kit’ in order to upgrade the trip system.
Circuit breakers having electronic trip units are well known in the art. Patented disclosures of such circuit breakers having electronic trip units may be found, for example, in U.S. Pat. Nos. 4,672,501; 6,678,135; and 6,534,991.
Commercially available circuit breakers are constructed to operate for decades in permanent electrical switchgear installations. The systems in which these circuit breakers operate are built to serve the electrical needs of the facility as envisioned at the time of their initial design. However, over time, these initial needs may often change, regulatory imperatives may often force modifications, or advances in protection technology in time may provide compelling reasons to update the switchgear's initial mission. Due to the size and complexity of a typical electrical switchgear installation, and the rugged nature of circuit breakers, it is rarely necessary, or economical, to replace the switchgear or breakers in order to modify or upgrade an electrical system's protection capabilities.
New advances in protection technology may be (and often are) deployed in existing switchgear by upgrading the trip units that control the breakers' operation. These electronic “brains” continually monitor the electrical conditions of the breaker and its attached loads, and will command the breaker mechanism to open if established electrical operating limits are violated.
As indicated above, the problem faced when upgrading circuit breaker trip units is that the mechanical and electrical interface between the circuit breaker mechanism and the trip unit often varies widely from breaker to breaker, even among breakers from the same manufacturer. Additionally, regulatory requirements permit only properly configured trip unit/breaker combinations. Complex mechanical and electronic “rejection” features are in place to prevent the installation of mismatched trip unit/breaker pairs. These rejection features are typically unique to each breaker and trip unit family, with thousands of possible permutations.
When a new trip unit is created and becomes commercially available, i.e., a unit offering newer features and better performance than earlier models, the job of matching the new device to the myriad of existing interfaces is daunting and time consuming. In short, different existing circuit breakers may contain a unique breaker interfaces designed only for that circuit breaker, and trip units designed for that specific unique interface may not be replaced by a trip unit designed for another specific circuit breaker.
Another problem known in the circuit breaker industry is one of counterfeit electronic trip units. By copying the form and circuitry of electronic trip units, counterfeiters develop and sell electronic trip units of poorer quality which are labeled falsely as manufactured by a known circuit breaker company. Mechanical features of rating plug and trip unit rejection schemes are easy to view and copy.
Presently, dozens of varieties of a basic trip unit may be required in order to satisfy the variety of breaker installations available and desirable for retrofit. Thus, there are still a number of drawbacks and deficiencies in currently utilized apparatus for circuit breaker technology for which additional technical advances are needed. The method and apparatus described herein address such an advance.
The aspects of the presently described invention will become more readily apparent to the reader with regard to the following figures and detailed description: